This project consists of a 20-year prospective study of the population-at-risk in osteoporosis. Research subjects are 170 normal women, aged 35-45, at the start of the study six years ago. Observations are made once every five years and consist of comprehensive calcium kinetic, calcium metabolic and physiological measurements as well as radiogrammetric measurement of bone mass. A total of 70 primary and derived variables is accumulated on each subject. The purposes of the project are: 1. To obtain comprehensive longitudinal data on calcium metabolism, bone metabolism, and related physiological functions in a group of untreated women from the perimenopausal period to the median onset time of osteoporotic symptoms; 2. Thereby to provide an exhaustive physiological profile both on women who develop symptomatic osteoporosis, and on those who do not, allowing characterization of the differences between them and of the factors influencing the differences; 3. Thereby permitting better identification of the osteoporosis-prone subset and providing both a rational basis for prophylactic treatment and the identification of subjects who should be its target; 4. And finally, to characterize cross-sectional interrelationships between major physiological variables during this key era of osteoporosis development.